It's the FlyTampa scenery. Very well done I thought.
As for frame rates, I'm at a bit of a loss to comment as I now find myself in the enviable situation of not having to worry about FPS any more.
I upgraded my computer to a bit of a monster (Dell XPS 710 with Nvidia 8800-768 MB) to have a go at FSX, but after trying FS9 on it I'm not budging.
I have my FPS locked at 40, with everything possible maxed out. A large collection of very detailed scenery, real weather, Flight and Ground Environment, etc. etc. and the FPS stays put at 40. Smooth as silk. Magic!
I have most of FlyTampa's scenery and can vouch for their excellent quality. Great animation too, but not overdone. I particularly like their Vienna (LOWW) scenery.
